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'Homeless' Planets May Be Common In Our Galaxy
sciencehabit writes "Our galaxy could be teeming with 'homeless' planets, wandering the cosmos far from the solar systems of their birth, astronomers have found. In a paper published online today in Nature, the researchers list 10 objects in our galaxy that are very likely to be free-floating planets. What's more, they claim that in our galaxy, free-floaters are probably so populous that they outnumber stars."
Apparently it's not very significant since not only are planets smaller than stars, they are smaller by a pretty large factor.
The mass of Jupiter is about 1/1000 solar masses. Let's say the average mass of these independently floating planets is about 10 times that of Jupiter, and that the average star is about the same as our Sun or less. That would make the mass of an average planet about 1/100 of the average star, so you'd still need planets to outnumber stars by a factor of 100 just to equal the mass of stars. Wikipedia says that visible matter makes up for about 17% of the total matter of the universe, so even if the mass of planets equaled that of stars (which, with the very very rough figures above, would mean a planet-to-star ratio of 100, or something pretty large anyway), there would still be plenty of dark matter to explain.
OK, so I've never really understood 'dark matter', but if there's a bunch of stuff floating about that's not stars and only shows up through things like gravitational micro-lensing ... might this cover some of the mass that is dark matter?
Maybe, for galactic dark matter, which is completely unrelated in every respect to dark matter on larger scales, although ignorant people typically use the general term "dark matter" to refer to all types of dark matter indiscriminately, creating enormous confusion in the process.
Galactic dark matter (GDM) is hypothesized as an explanation for the flat rotation curves of spiral galaxies. Based on the visible matter (stars) in a galaxy we can get an estimate of the mass inside a given radius. At sufficiently high radii we see the amount of visible matter dropping off, and expect that the few stars at even larger radii will start to behave like planets orbiting a distant mass with a 1/r**2 fall off in gravitational strength. But we don't see that. Instead more distant stars move as if the amount of matter inside their orbits around the galactic center contained ever more mass as they get further and further away. We can't see any visible matter to account for this, ergo, "dark matter".
One possible candidate for GDM are so-called "MACroscopic Halo Objects" (MACHOS, to contrast them with Weakly Interacting Massive Particles, or WIMPS. Physicists really need to get out more.)
An impediment to the MACHO hypothesis has been that the Initial Mass Function, which describes the probability of an object of mass M condensing out of a primordial cloud of gas and/or dust, was believed to drop off rather steeply at low masses. This observation suggests that it is at least a little higher than previously estimated, although I don't know if that is anywhere near high enough to account for a significant portion of GDM--my sense is not, but it's been a few years since I've paid much attention to this question.
At larger scales we also see anomalous motion of galaxies and galactic clusters relative to the amount of visible matter, and at the very largest scale there is much less visible mass than required to keep the universe in the state of almost-but-not-quite-closed that we see. If these phenomena are caused by an excess of matter at larger scales we know that it is non-baryonic (not made of protons and neutrons) because we have a very good estimate of the density of protons and neutrons in the universe based on primordial nucleosynthesis: the denser the early universe was in protons and neutrons, the more helium would have been created, and given we know the early universe was about 23% helium (there are complex self-consistency checks on this number based on other atomic species) we know there are not enough protons and neutrons to account for the large-scale dark matter (LSDM).
Therefore, we know that LSDM is completely unrelated in every respect to GDM: the problems they solve have different constraints and one requires exotic new physics while the other is relatively mundane. It is deeply unfortunate that people are so incompetent in their use of abstractions that they are chronically unable to distinguish between these two unrelated problems.
Blasphemy is a human right. Blasphemophobia kills.
Read Fritz Lieber's "The Wanderer".
It discusses exactly this.
Ok, so I actually have an advanced degree in astrophysics. While you do describe the basic observations that lead us to believe dark matter exists, it's not true that galactic dark matter and large scale dark matter are different beasts. True, MAssive Compact Halo Objects (such as rogue black holes, neutron stars, brown dwarfs, etc) (note the correction) were a possible explanation, but we've done observational studies that look for them using microlensing, and although we did find a few, there wasn't nearly enough (i.e. several orders of magnitude less) to explain our galactic rotation curve. WIMPS (such as neutrinos) have been ruled out since they fail to explain the observed large scale galactic structure, and there aren't nearly high enough neutrino counts in neutrino observatories to make them a viable option.
It turns out that, for BOTH galactic rotation curves and large scale darkmatter, you need about 10 times more mass than what we can associate with stars, so the two problems actually do have the same constraints. Therefore, it's very likely that there is some form of matter which only interacts gravitationally (and not electromagnetically: i.e. with light) with normal baryonic matter which has so far been unobserved. (Not surprising, since a lot of our matter detection techniques rely on interactions with light, and besides the required density of this stuff would make it very rare on Solar System scales - it only becomes significant on galactic scale interactions).
On the original article - It's not too surprising that there are lot of free roaming planets, it just indicates that there was a higher degree of fragmentation in molecular clouds than was once thought. However, it would require an insanely HUGE number of them to explain dark matter observations - planets are generally much less massive than stars, somewhere around 10^6 to 10^8 times less massive, so there would have to be 10's of billions times more of them than stars to explain dark matter observations, something that the article does not assert is true.